This invention relates generally to steam turbine buckets (or blades) and, more particularly, to the adhesion of filler material in hybrid or composite blades.
Steam turbine blades operate in an environment where they are subject to high centrifugal loads and vibratory stresses. Vibratory stresses increase when blade natural frequencies become in resonance. The magnitude of vibratory stresses when a blade vibrates in resonance is proportional to the amount of damping present in the system (damping to a smaller or greater degree is achieved via materials and the aerodynamic and mechanical components), as well as the vibration stimulus level.
At the same time, centrifugal loads are a function of the operating speed, the mass of the blade, and the radius from engine centerline where that mass is located. As the mass of the blade increases, the physical area or cross-sectional area must increase at lower radial heights to be able to carry the mass above it without exceeding the allowable stresses for the given material. This increasing section area of the blade at lower spans contributes to excessive flow blockage at the root and thus lower performance. The weight of the blade also contributes to higher disk stresses and thus potentially to reduced reliability.
Several prior U.S. patents relate to so-called “hybrid” blade designs where the airfoil portion of the metal blade is formed with one or more pockets filled with a polymer (or polymer/metal, glass or ceramics mix) filler material. These prior patents include U.S. Pat. Nos. 6,287,080; 6,139,278; 6,042,338; 6,039,542; 6,033,186; 5,947,688; 5,931,641 and 5,720,597. See also co-pending commonly owned application Ser. No. 10/249,518, filed Apr. 16, 2003. One area not addressed by the prior work in this area is the problem of achieving more reliable adhesion of the filler within the pocket or pockets formed in the airfoil portion of the blade.
More specifically, the large incidence angles of steam flow to the bucket surface could cause the cast polymer filler to delaminate from the pocket formed in the airfoil portion of the blade. In other words, the large angle of incidence of the steam flow to the bucket surface exposes a higher risk of the flow tending to “lift” the filler material off the pocketed surface.